Advances in managing long-bone fractures (Proceedings)

The most significant change in recent years in the management of fractures has been the more recent emphasis on "biologic fixation" as opposed to "mechanical fixation". The latter refers to the direct fixation techniques whereby fractures are reconstructed anatomically, and the former refers to indirect fixation techniques whereby the fractures are spanned (or "bridged"). The phrase "biological fracture fixation" is a philosophic change towards a more "flexible" fixation with less precise, indirect methods of fracture reduction. The importance of the blood supply to the fracture – and the preservation of the vasculature under the plate – also has been emphasized.

There are many fractures whereby there is either a large gap due to missing bone, or a similarly large gap spanned by significant comminution. In either case the fracture gap cannot (or should not) be reconstructed. The primary aspect to consider with some form of buttress/bridge plating is that the soft-tissues have already undergone a great deal of compromise; therefore, techniques that add to this compromise (e.g., excessive surgical dissection, and thus a prolong surgical time) will result in additional vascular compromise that leads to decreased osteogenesis, with the added consequence of impaired stability and increased infection rates. In such instances the "direct reduction techniques" are replaced with "indirect reduction techniques". The former embraces an anatomic (mechanical) reconstruction, whereas the latter embraces a more biologic reconstruction. The indirect techniques have been promoted as "open, but do not touch (OBDNT)", and more recently the minimally invasive techniques. In these indirect techniques there is a more limited stability to the fixation – the bone no longer shares any of the load with the implants, re: buttress/bridging fixation, but there is better preservation (the emphasis on biology) of the vascular (soft-tissue) envelope; the tissues are spared further trauma by a closed or "OBDNT" approach. Thus the bone healing capability is enhanced.

There is a direct trade-off between the direct vs. indirect surgical techniques. This is the "concept of balance", or a trade-off between mechanics & biology. This balance may be shifted towards one side or the other, where the absence of one requires more of other, but within reasonable limit. The absolute absence of either factor results in a lack of healing; thus, the proposed requirements for utilizing each of these techniques must be fully appreciated.

Bone plates function in a buttress (or bridging) fashion when a gap exists at the fracture site such that the bone fragments do not share in load bearing. The plate supports the fractured bone, maintains normal limb length, and prevents collapse at the fracture site. The plate bears the entire functional load applied through the bone until healing occurs. This technique provides stability at the fracture site with minimal disruption of the fracture hematoma and soft tissues surrounding the fracture.

Clinical indications – buttress plate fixation

This method of fixation is used to stabilize large fracture gaps, e.g., highly comminuted non-reconstructable fractures, infection, non-union, malunion or angular limb deformities (with re-establishment of limb length), or bone resection due to neoplasia.

Various types of plates can be used in buttress fashion, including standard plates, limb-lengthening plates, and more recently the locking plates. As an example using the Synthes® implants, there is more uniform bending of the LC-DCP® (as compared to the DCP® where stress-risers exist at the open screw holes); therefore, this plate type is preferred. Screw placement is modified such that they are placed at the far ends of the screw hole ("buttress position"); thus positioning the screw near the steep wall of the plate hole (as opposed to the neutral or compression position). This rigidly fixes the plate and screw to minimize collapse of the fracture gap. Despite these plates improved uniform bending characteristics (as compared to standard plates), the empty screw holes present over any bone void may still result in a local area of weakness (stress riser) where the plate may subsequently break. One method to offset this problem is to utilize >1 plate, either placed opposite or orthogonal to the first plate, thus minimizing the stress placed on a single device. The limitation to this approach has been the ability to secure a sufficient number of bicortical screws to secure the plates (interference is present between the screws, and/or plate, that oppose each other). This problem recently has been minimized with the advent of the locking plate systems where monocortical screws may be placed.

Because of the limitation with open screw holes when using a single plate, an alternate plate design has been manufactured for spanning gaps: the limb lengthening plates, which are specifically designed for use in buttress fashion. The central portion of the plate is devoid of holes to eliminate this potential stress riser and weakness. There are some concerns, however, that these plates are too stiff. They have largely been replaced by the plate/rod technique, and more recently by the locking plates.

A similar philosophy has been used to bridge fractures with an intramedullary (IM) pin, i.e., the interlocking nail. In this form of fixation, direct access to the fracture site is avoided and the implant spanned across the fracture. To provide both resistance against rotational and compressive forces, the IM rod is locked with screws/bolts that penetrate the rod. Some limitations include position of the fracture in the bone diaphysis and the recognition that some slight rotational instability remains present (termed "slack") as these screws/bolts do not engage directly into the rod, but instead penetrate it and are secured in the bone cortex.

Clinical indications – plate-rod fixation

From a mechanical perspective, the implant system must limit the strain to a level that will continue to permit bone union. Comminuted fractures distribute strain over a large surface area, which lowers interfragmentary strain to a level compatible with direct or indirect bone union. However, if the bony column is not reconstructed, an implant is placed under considerable stress since it must carry the entire physiologic load until callus (bio-buttress) is formed. If a standard bone plate is used, empty plate holes will be present overlying the area of comminution. In that an empty plate hole serves as a stress concentrator, plate failure can occur in this area. One method to reduce plate strain is to combine the bone plate with an addition of an IM pin. This technique involves the combination of an intramedullary (IM) Steinmann pin and a plate used in concert. This technique, in addition to providing additional stability to the bone, facilitates reconstruction. The IM pin not only adds to the strength of the repair, but allows for alignment and length correction to be performed with ease before plate placement. The major remaining requirement of appropriate rotational alignment requires appropriate knowledge of the spatial arrangements and anatomic keys when aligning such a non-reconstructable fracture.

In performing this technique, an IM pin is first inserted normograde to restore proper bone length and alignment, and to maintain distraction while the plate is applied (often the pin tip is cut once the pin is located at the fracture site; in this manner there is less risk that the pin will traverse the distal bone fragment when used to re-establish bone length).

This IM pin only fills 35-40% of the medullary cavity. An IM pin that occupies 40% the diameter of the marrow cavity reduces the stress on the primary implant (plate) by 50% or more. More importantly, this extends the fatigue life of the primary implant at least 10-fold. Alternatively, a smaller IM pin that only occupies 25% of the marrow cavity only reduces the stress in the primary implant by a factor of 10%; therefore, appropriate pin size is critical. The smaller pin size (as compared to primary IM pin fixation, which fills ~2/3 of the medullary canal diameter) provides ample room to insert the screws for plate fixation without interference, and also and eliminates any excessive rigidity to the plate/rod construct if a much larger pin would be used.

The bone fragments and fracture hematoma are not disturbed; however, if large fragments are far from the bone column, they may be brought closer to the bone shaft, better approximating overall alignment and subsequent healing, using absorbable suture material (i.e., some re-apposition of the soft-tissue envelope only). No attempt is made to anatomically reduce the fragments. A bone plate then is contoured to the surface of the bone and applied in buttress fashion. Radiographs of the contralateral normal bone may be helpful when contouring this plate. The most proximal and distal screws are inserted to engage both cortices of the bone (bicortical). There is ample flare of the metaphysis/epiphysis where all screws can be placed bicortically. The remaining screws need only engage the near cortex (monocortical). A minimum of three monocortical screws and one bicortical screw should be used on each side of the fracture. The presence of the IM pin reduces strain on the plate and significantly increases the plate's fatigue life, as noted above.

Clinical indications – interlocking nail

As noted, these are used for diaphyseal fracture fixation. Sufficient intact bone must be present at either bone end (metaphysis) such that there is an ability to secure the screws/bolts in intact cortex (that also passes through the rod) at both bone ends. Additionally, this device is only indicated for fractures with low interfragmentary strain, i.e., multiple fragmentation and/or large gaps. Small, single gaps have high strain present; therefore, any movement at the fracture gap (due to the inherent "slack" of this device) produces high strain levels and inhibits healing.

Bone vascularity

Porosity of bone under a plate was originally thought to be as a result of stress protection; however, the shape of the area of the porosity was shown not to correlate with the pattern of unloading provided by the plate; additionally, the transient nature of this phenomena did not equate with the continued "protection" (stress protection) assumed to be provided by the plate. Plates with reduced contact had less porosity, and plates with more flexibility (and thus greater bone contact) had greater temporary porosity. These findings were re-assessed to be the result of bone remodeling secondary to bone necrosis – as a result of the loss of the vascular supply, and not remodeling changes consistent with Wolff's law. As a result, the emphasis turned to minimal plate contact with the bone so as to preserve the vascular supply under the plate. In addition, the more flexible fixation applied through indirect reduction was consistent with the strain theory (described by Perren) whereby small unstable gaps were avoided. Similarly, the indirect reduction methods provided a minimum of biologic interference at the time of implant application. This flexibility of fixation was attained by bridging the fracture gap at greater distances and decreasing the size and material properties of the implant. To preserve the stability at the bone-implant interface the locked construct was developed, which also minimized the risk of loosening and fretting abrasion possible at the screw-plate interface; the latter also was minimized by beginning to use more corrosive-resistant materials such as Ti. Finally, the preservation of the vascularity was shown to improve the local resistance to infection by the preservation of the vascularity under the plate (in a rabbit model was shown to be 1:750).

Clinical indications – locked plating

Bridge plating also can be effectively performed using the locked plating systems. Because these plates have screws that lock to the plate, a fixed angle construct is created – identical to an external skeletal fixator, albeit as an internal device. A fixed angle construct eliminates the need to contour the plate exactly to the bone surface (standard plates require compression of the plate to the bone in order to obtain a "mixed mode" of force transfer, i.e., utilizing the frictional forces between the plate and bone in order to obtain construct stability), as loads are maintained with a "screw-only" mode of force transfer. This aspect permits the minimally invasive fixation techniques to be more successfully utilized, where plates can be inserted percutaneously. Additionally, the fixed angle constructs allow fewer screws to be utilized; e.g., epiphyseal fractures.

Regardless, the limitation of a single plate spanning a gap needs to be appreciated due to the lack of any load-sharing with the bone, as the plate may be subject to overloading (see above: rationale for the plate-rod technique). For this reason, a supplemental IM pin or a second plate should be considered to provide additional support. A second plate (opposite or orthogonal to the first plate) can be applied without concern for screw interference since monocortical screw fixation may be used. Because these plates are not compressed to the bone surface, there is additional sparing of the vasculature (under the plate) that permits application of multiple fixation devices without compromising the biology.

This approach again favors the preservation of the soft-tissue envelope. Despite this apparent advantage using minimally invasive techniques, they should be used judiciously, as the mechanical advantage of a plate is lessened with increasing distance present between the plate and bone. Furthermore, anatomic knowledge of the unseen soft-tissues is imperative. Similarly, limited screw purchase of a single device (limited screw purchase with a periarticular fracture, for example) may not be of sufficient strength to neutralize the applied loads, and additional support (a second device) may be necessary. Although these techniques have been used successfully in human orthopedics, they need to be applied with caution in animals where immediate full weight-bearing postoperatively is the expectation.

References

Arens S, Schlegel U, Printzen C, et al: Influence of materials for fixation implants on local infection: An experimental study of steel versus titanium in rabbits. J Bone Joint Surg [Br] 78B:647-651, 1996

Arens S, Eijer H, Schlegel U, et al: Influence of the design for fixation implants on local infection: experimental study of dynamic compression plates versus point contact fixators in rabbits. J Orthop Trauma. 13:470-476, 1999

Boudrieau RJ. Locking plates: More questions than answers. Proceedings of the 2007 ACVS Veterinary Symposium. Chicago, Illinois. October 18-21, 2007; pp 391-394

Boudrieau RJ. The Advanced Locking Plate System (ALPS): Design rationale, biomechanics and early clinical use. Proceedings of the 2009 ACVS Veterinary Symposium. Washington DC; October 8-10, 2009; pp 482-486

Déjardin L. I-LOC – A Novel angle-stable interlocking nail implant. Proceedings of the 2009 ACVS Veterinary Symposium. Washington DC; October 8-10, 2009; pp 491-495

Gautier E, Rahn BA, Perren SM: Vascular remodelling. Injury 29 (Suppl 2):S-B11-19, 1995

Haidukewych GJ, Ricci W: Locked plating in orthopaedic trauma: a clinical update. [Review] J Am Acad Orthop Surg 16:347-355, 2008

Hulse D, Hyman W, Nori M, et al. Reduction of plate strain by addition of an intramedullary pin. Vet Surg 26:451-459, 1997

Hulse D, Ferry K, Fawcett A, Gentry D, et al. Effects of intramedullary pin size on reducing bone plate strain. Vet Comp Orthop Trauma 13:185-190, 2000

Kubiak EN, Fulkerson E, Strauss E, et al: The evolution of locked plates. J Bone Joint Surg [Am] 88A(Suppl 4):189-200, 2006

Miller CW, Kuzma AB, Sumner-Smith G. The narrow lengthening plate for treatment of comminuted diaphyseal fractures in dogs. Vet Comp Orthop Trauma 3:108-112, 1989

Perren SM. Evolution of the internal fixation of long bone fractures. Review]. J Bone Joint Surg [Br] 84B:1093-1110, 2002

Schatzker J, Meutstege F, Prieur WD. Implants and their application. In: Brinker WO, Olmstead ML, Sumner-Smith G, Prieur WD, eds. Manual of Internal Fixation in Small Animals. Berlin: Springer-Verlag, 1998, 57-96

Walter MC, Lenehan TM, Smith GK, et al. Treatment of severely comminuted diaphyseal fractures in the dog using standard bone plates and autogenous cancellous graft to span fracture gaps: 11 cases (1979-1983). J Am Vet Med Assoc 189:457-462, 1986

Zura RD, Browne JA: Current concepts in locked plating. J Surg Orthop Adv 15:173-176, 2006